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# Quickstart

The quick start will cover the basics of working with language models. It will introduce the two different types of models - LLMs and ChatModels. It will then cover how to use PromptTemplates to format the inputs to these models, and how to use Output Parsers to work with the outputs. For a deeper conceptual guide into these topics - please see [this documentation](./concepts)

## Models
For this getting started guide, we will provide a few options: using an API like Anthropic or OpenAI, or using a local open source model via Ollama.

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First we'll need to install their partner package:

```shell
pip install langchain-openai
```

Accessing the API requires an API key, which you can get by creating an account and heading [here](https://platform.openai.com/account/api-keys). Once we have a key we'll want to set it as an environment variable by running:

```shell
export OPENAI_API_KEY="..."
```

We can then initialize the model:

```python
from langchain_openai import ChatOpenAI
from langchain_openai import OpenAI

llm = OpenAI()
chat_model = ChatOpenAI()
```

If you'd prefer not to set an environment variable you can pass the key in directly via the `openai_api_key` named parameter when initiating the OpenAI LLM class:

```python
from langchain_openai import ChatOpenAI
llm = ChatOpenAI(openai_api_key="...")
```

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[Ollama](https://ollama.ai/) allows you to run open-source large language models, such as Llama 2, locally.

First, follow [these instructions](https://github.com/jmorganca/ollama) to set up and run a local Ollama instance:

* [Download](https://ollama.ai/download)
* Fetch a model via `ollama pull llama2`

Then, make sure the Ollama server is running. After that, you can do:
```python
from langchain_community.llms import Ollama
from langchain_community.chat_models import ChatOllama

llm = Ollama(model="llama2")
chat_model = ChatOllama()
```

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First we'll need to import the LangChain x Anthropic package.

```shell
pip install langchain-anthropic
```

Accessing the API requires an API key, which you can get by creating an account [here](https://claude.ai/login). Once we have a key we'll want to set it as an environment variable by running:

```shell
export ANTHROPIC_API_KEY="..."
```

We can then initialize the model:

```python
from langchain_anthropic import ChatAnthropic

chat_model = ChatAnthropic(model="claude-2.1", temperature=0.2, max_tokens=1024)
```

If you'd prefer not to set an environment variable you can pass the key in directly via the `anthropic_api_key` named parameter when initiating the Anthropic Chat Model class:

```python
chat_model = ChatAnthropic(anthropic_api_key="...")
```

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First we'll need to install their partner package:

```shell
pip install cohere
```

Accessing the API requires an API key, which you can get by creating an account and heading [here](https://dashboard.cohere.com/api-keys). Once we have a key we'll want to set it as an environment variable by running:

```shell
export COHERE_API_KEY="..."
```

We can then initialize the model:

```python
from langchain_community.chat_models import ChatCohere

chat_model = ChatCohere()
```

If you'd prefer not to set an environment variable you can pass the key in directly via the `cohere_api_key` named parameter when initiating the Cohere LLM class:

```python
from langchain_community.chat_models import ChatCohere

chat_model = ChatCohere(cohere_api_key="...")
```

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Both `llm` and `chat_model` are objects that represent configuration for a particular model. 
You can initialize them with parameters like `temperature` and others, and pass them around.
The main difference between them is their input and output schemas.
The LLM objects take string as input and output string.
The ChatModel objects take a list of messages as input and output a message.
For a deeper conceptual explanation of this difference please see [this documentation](./concepts)

We can see the difference between an LLM and a ChatModel when we invoke it.

```python
from langchain_core.messages import HumanMessage

text = "What would be a good company name for a company that makes colorful socks?"
messages = [HumanMessage(content=text)]

llm.invoke(text)
# >> Feetful of Fun

chat_model.invoke(messages)
# >> AIMessage(content="Socks O'Color")
```

The LLM returns a string, while the ChatModel returns a message.

## Prompt Templates

Most LLM applications do not pass user input directly into an LLM. Usually they will add the user input to a larger piece of text, called a prompt template, that provides additional context on the specific task at hand.

In the previous example, the text we passed to the model contained instructions to generate a company name. For our application, it would be great if the user only had to provide the description of a company/product without worrying about giving the model instructions.

PromptTemplates help with exactly this!
They bundle up all the logic for going from user input into a fully formatted prompt.
This can start off very simple - for example, a prompt to produce the above string would just be:

```python
from langchain.prompts import PromptTemplate

prompt = PromptTemplate.from_template("What is a good name for a company that makes {product}?")
prompt.format(product="colorful socks")
```

```python
What is a good name for a company that makes colorful socks?
```

However, the advantages of using these over raw string formatting are several.
You can "partial" out variables - e.g. you can format only some of the variables at a time.
You can compose them together, easily combining different templates into a single prompt.
For explanations of these functionalities, see the [section on prompts](/docs/modules/model_io/prompts) for more detail.

`PromptTemplate`s can also be used to produce a list of messages.
In this case, the prompt not only contains information about the content, but also each message (its role, its position in the list, etc.).
Here, what happens most often is a `ChatPromptTemplate` is a list of `ChatMessageTemplates`.
Each `ChatMessageTemplate` contains instructions for how to format that `ChatMessage` - its role, and then also its content.
Let's take a look at this below:

```python
from langchain.prompts.chat import ChatPromptTemplate

template = "You are a helpful assistant that translates {input_language} to {output_language}."
human_template = "{text}"

chat_prompt = ChatPromptTemplate.from_messages([
    ("system", template),
    ("human", human_template),
])

chat_prompt.format_messages(input_language="English", output_language="French", text="I love programming.")
```

```pycon
[
    SystemMessage(content="You are a helpful assistant that translates English to French.", additional_kwargs={}),
    HumanMessage(content="I love programming.")
]
```


ChatPromptTemplates can also be constructed in other ways - see the [section on prompts](/docs/modules/model_io/prompts) for more detail.

## Output parsers

`OutputParser`s convert the raw output of a language model into a format that can be used downstream.
There are a few main types of `OutputParser`s, including:

- Convert text from `LLM` into structured information (e.g. JSON)
- Convert a `ChatMessage` into just a string
- Convert the extra information returned from a call besides the message (like OpenAI function invocation) into a string.

For full information on this, see the [section on output parsers](/docs/modules/model_io/output_parsers).

In this getting started guide, we use a simple one that parses a list of comma separated values.

```python
from langchain.output_parsers import CommaSeparatedListOutputParser

output_parser = CommaSeparatedListOutputParser()
output_parser.parse("hi, bye")
# >> ['hi', 'bye']
```

## Composing with LCEL

We can now combine all these into one chain.
This chain will take input variables, pass those to a prompt template to create a prompt, pass the prompt to a language model, and then pass the output through an (optional) output parser.
This is a convenient way to bundle up a modular piece of logic.
Let's see it in action!

```python
template = "Generate a list of 5 {text}.\n\n{format_instructions}"

chat_prompt = ChatPromptTemplate.from_template(template)
chat_prompt = chat_prompt.partial(format_instructions=output_parser.get_format_instructions())
chain = chat_prompt | chat_model | output_parser
chain.invoke({"text": "colors"})
# >> ['red', 'blue', 'green', 'yellow', 'orange']
```

Note that we are using the `|` syntax to join these components together.
This `|` syntax is powered by the LangChain Expression Language (LCEL) and relies on the universal `Runnable` interface that all of these objects implement.
To learn more about LCEL, read the documentation [here](/docs/expression_language).

## Conclusion

That's it for getting started with prompts, models, and output parsers! This just covered the surface of what there is to learn. For more information, check out:

- The [conceptual guide](./concepts) for information about the concepts presented here
- The [prompt section](./prompts) for information on how to work with prompt templates
- The [LLM section](./llms) for more information on the LLM interface
- The [ChatModel section](./chat) for more information on the ChatModel interface
- The [output parser section](./output_parsers) for information about the different types of output parsers.